Methods based on magnetic resonance, such as magnetic resonance tomography (MRS) or spectroscopy (MRS), require adapted and good physical ambient conditions to provide optimum quality of the recorded data. This applies by way of example to the spatial homogeneity, stability over time, and the absolute accuracy of the associated magnetic fields, (e.g., of the B0 field, which is provided to generate the magnetization, and of the B1 field with which the magnetization is tilted out of equilibrium). The measurement object located in the B0 field obtains magnetization by way of the basic magnetic field B0, and for the detection of measuring signals this magnetization is disrupted by the emission of high frequency pulses (HF pulses), the B1 field. The magnetization that returns to state of equilibrium is spatially encoded during imaging by the connection of encoding magnetic field gradients and is received by one or more receiving coil(s). A HF pulse generates an amplitude-modulated B1 field oscillating with a carrier frequency, and this is oriented perpendicularly to the B0 field. The angle of tilt a describes the tilting of the magnetization from the state of equilibrium and influences contrast and signal intensity of the received signals. If a desired angle of tilt is not attained during a resonance excitation, then this leads to contrast and signal losses.
The B1 field generated by a HF pulse depends not just on a controllable initial voltage of a HF unit, which generates the HF pulse, but also on a load dependent on the measurement object, which depends on the examination object/measurement object. For this reason, for accurate determination of the angle of tilt, the initial voltage of the high frequency amplifier is determined for each examination object and for each position of the examination object in the basic magnetic field in one alignment. The initial voltage generates a specific B1 field and therewith a desired angle of tilt for a standardized HF pulse. Static field disruptions and spatial variations in the HF field due in particular to the measurement object and susceptibility may be taken into account. These measurement object-dependent alignments are determined before the actual measurement, and this may be an imaging measurement or a spectroscopic measurement. If the boundary conditions, such as the table position, e.g., the position of the measurement object in the magnet, or the choice of transmitting and receiving coils, change then the measurement object-dependent alignments have to be carried out again.
In addition to this alignment of the initial voltage mentioned above, the transmitter alignment, a frequency alignment is also carried out in which the HF carrier or center frequency is configured to the resonance frequency of the excited nuclei being considered. A further, third alignment is conventionally carried out twice, namely one alignment in which a voltage measured at a directional coupler directly behind the HF amplifier is compared with the voltage effectively applied at the transmitting coil. During this alignment, the measured values are recorded and other components of the MR facility provided. This last alignment is conventionally carried out twice, namely once before the frequency and transmitter alignment to determine rough characteristic values and a second time thereafter by taking into account the results of the frequency and transmitter alignment.
If the actual MR measurement, for which the alignments are carried out, occurs in a fixed position of the measurement object in the MR facility, then the stationary alignments are conventionally carried out in this fixed position.
Alternatively, for several years, it has been possible to also record MR data in the case of a moving measurement object. Measurement object-dependent alignments are also carried out in this connection by moving the entire region of the measurement object to be examined through the isocenter of the magnet of the MR facility. The alignment characteristic values are determined in a grid of several centimeters, so that the measurement object-dependent alignments do not have to be repeated again later for changing examination positions. The quality of the alignment results determined in the case of moving measurement objects is much poorer, however, since with continuous movement of the measurement object no iteration is carried out, as in the case of the stationary alignment measurements. The results of the alignment measurement in the case of a moving measurement object may not replace the stationary alignments therefore.
The iterative methods used in the case of the stationary alignments are very time-consuming and are regarded as disruptive by the user.